In various multi-carrier modulation systems, e.g. Orthogonal Frequency Division Multiplexing (OFDM) or Discrete Multi-tone (DTM), multi-carrier signals suffer from a high peak to average power ratio. A large Peak-to-Average-Ratio (PAR) brings disadvantages like a reduced efficiency and lower average output power of a power amplifier (PA). The objective of PAR reduction techniques, such as clipping or decresting, is therefore to reduce the peak amplitude excursions of the output signal while keeping the spectrum expansion within specified limits, such as spectral mask and adjacent channel power ratio (ACPR) specifications, and keeping in-band error within specified limits, so-called error vector magnitude (EVM) specification. PAR reduction thus increases efficiency and average output power of a peak power limited power amplifier.
There are many existing prior art solutions dealing with PAR reduction for multi-carrier signals e.g. OFDM signals.
A well known prior art approach for reducing the peak power of a multi-carrier signal is to implement Tone Reservation, also known as Tone Reduction. In this method, described in J. Tellado-Mourello, “Peak to Average Reduction For Multicarrier Modulation” Dept. of Electrical Engineering of Standford University, pp. 66-99, September 1999, the peak power is reduced by selecting or reserving a subset of a plurality of frequencies or tones that make up a multi-carrier symbol. These selected or reserved frequencies are used to create an approximate impulse function, also known as a kernel.
FIG. 1 illustrates the prior art technique for reducing peak power of an input main signal using a kernel signal. In FIG. 1 an input multi-carrier signal composed of {X0, X1, . . . XN-1} originally in a frequency domain is converted into a time-discrete domain signal denoted x(n) using an N-point Inverse Fast Fourier Transformer. N is the number of sub-carriers of the original input signal and N can take any value depending on the desired data rate and other requirements on the system which the apparatus is to be integrated with. As can be seen in FIG. 1, some sub-carriers Xi are equal to zero. These sub-carriers are the reserved tones used to reduce the peak power of the system. These reserved sub-carriers or tones are usually not used for data transmission instead they are reserved for anti peak signals and they are orthogonal to the other tones which carry data. The reserved tones are further used to construct a reduction signal {C1, C2, . . . CN-1} which is further passed through an N-point Inverse Fast Fourier Transformer in order to generate a time-discrete domain signal c(n) of similar size as x(n), i.e. having the same number of samples as x(n), and adding this signal c(n) to the original time domain signal x(n) to cancel large peaks. This tone reservation technique restricts the data block {X0, X1, . . . XN-1} and peak reduction block signal {C1, C2, . . . CN-1} to lie in disjoint frequency subspaces i.e. XkCk=0. This is illustrated in FIG. 1 where {C1, C2, . . . CN-1} has zero values when {X0, X1, . . . XN-1} has non-zero values and vice versa.
An exemplary process of reducing a single peak of x(n) exceeding a threshold level A will now be described:
An appropriate kernel k(n) is constructed from peak reduction frequencies or similarly from the reserved frequencies described above.
This kernel k(n) is further scaled at a peak time value τ using a scaling factor Δ, and rotated such that its phase matches the phase of the overshooting part exceeding threshold level A.
The scaling factor Δ corresponds to the magnitude of the overshooting part exceeding a threshold level A, and τ corresponds to the peak time-discrete value.
Now, to reduce the peak of x(n) at time τ, c(1) is constructed according to: c(1)=A1(Δ)·k(n−τ), where A1(Δ) is a scaling factor greater than Δ such as for example 1.3Δ. Thus, when x(n) and c(1) are added at n=τ would the maximum value be A−1.3Δ, which results in a value less than the threshold level, and the peak has therefore been reduced.
The tone reservation technique described above repeatedly applies the kernel to cancel the peaks of the input signal. Thus, any number of peaks may be clipped in this fashion and in a single iteration. However, reducing one or more peaks may cause the resulting waveform to exceed maximum threshold value A at other sample positions. Therefore, the process may be repeated until a desired peak power is reached.
The kernel created from the subset of reserved frequencies are usually pre-computed since the subset of reserved frequencies is usually known in advance. The method described above is usually performed within each multi-carrier symbol (block). In case the multi-carrier signal is an OFDM signal, the above described PAR reduction method is usually performed within each OFDM symbol, before application of cyclic prefixes (or guard bands) and other transmit processing operations.
The basic idea of reducing the peak power using a kernel created from reserved frequencies of the multi-carrier signal is attractive and does really reduce the peak power, especially if the percentage of reserved frequencies is high. However, a high percentage of reserved frequencies impairs the channel since more bandwidth is lost for peak reduction purposes. Furthermore, the power in the reserved frequencies or tones adds to the average signal power without contributing to the data transfer. For this reason, it is beneficial to use a low percentage of reserved frequencies for certain channel conditions. For example, if the signal to noise ratio (SNR) of a channel is high, there is no need to use a high percentage of reserved frequencies.
In addition to the percentage of frequencies or tones that are used, the distribution of the tones is also important. In practical designs, generally random or irregular distributions of the reserved tones perform much better than evenly or regularly spaced tones or tones clustered in blocks. Although the best tone distributions are irregular or random, the error energy or clipping error and local bandwidth impairment will be unevenly distributed across the system bandwidth.
This uneven distribution of local bandwidth impairment is especially noticeable for systems wherein the available system bandwidth is divided into contiguous portions of frequencies, whereas varying number of portions are assigned to users. In such systems, some portions will have one or several reserved frequencies in it and some will not. For those users that are assigned portions that have different numbers of reserved tones in it will therefore experience different qualities of service i.e. degradation in quality of service and lower data transfer date since several reserved tones can be a significant part of the data-carrying capacity.